Oxides extracted from vegetal matter and process therefor

ABSTRACT

The invention concerns a process for the extraction of acid or basic oxides contained in a vegetal matter, more specifically it concerns the extraction of silica from rice husks. The invention also concerns pure oxides extracted from vegetal matter. The invention also concerns the process for the extraction of carbon-rich oxide compositions from vegetal matter, and compositions obtained through said process.

The present invention concerns pure oxides (acid or basic oxides)extracted from vegetal matter, as well as carbon-rich oxide compositionsextracted from vegetal matter. Another aspect of the invention concernsa process for the extraction of oxides contained in vegetal matter; morespecifically it provides a process for the extraction of silica fromrice husks or from the rice plant.

The description that follows makes reference to a preferred embodimentof the invention, namely the extraction of silica from rice husks. Thisis done simply to favor the understanding of the invention, withoutimposing any limitation to the scope of the invention, defined in theattached claims.

Amorphous silica, SiO₂, under its non crystalline form, is a substancethat has many uses. Its commercial value is strongly tied to its purity,specific area and particle size. Particles between 10 and 1000 nm(nanometers) are know as nanoparticles, and due to their small radiushave high chemical reactivity and sinterization capacity.

Among the known processes for obtaining amorphous silica are thereaction between silicon monoxide, SiO and oxygen, and the mere burningof rice husks by the grain companies. This last case leads to a productknown as rice husk ash.

The ash from the burning of rice husks contains high content of blackcarbon, about 60% by weight of silica, and some impurities mainlypotassium, sodium, magnesium and calcium. The elimination of the carbonfrom this mixture to obtains high purity silica requires the use of hightemperatures, a procedure that leads to BET specific areas of about 10m²/g and average particle size above 50 μm. The dry rice husk masscorresponds to about 20% of the in natura grain, a variable valueaccording to the plant variety, the climate, and agricultural proceduresemployed with the plant culture.

Rice husks comprise mainly cellulose, lignin, hemicellulose, silica andother inorganic oxides, these last ones representing about 4.5% of thesilica mass, potassium and calcium having the highest content. Suchvalues depend on the plant variety, climate and rain distribution.

Known in the state of the art is a process for the extraction of silicafrom rice husk developed by Real et. ali. (C. Real, M. D. Alcalá and J.M. Criado, “Preparation of Silica from Rice Husks” J. Am. Ceram. Soc. 79[8] 2012-12 (1996)). Such a process consists of boiling the rice husksfor two hours in a 10% hydrochloric acid distilled water solution, underambient pressure, at a temperature of no more than 100° C. The husks arethen washed with distilled water to eliminate salts and compounds ofpotassium, sodium, calcium and magnesium that contribute to the lateraggregation of particles of amorphous silica during calcination.Calcination at 600° C. follows, during an unknown amount of time,resulting in a white, amorphous and free from carbon silica, with a lowcontent of other inorganic compounds, specific area of 260 m²/g, a nonspecified particle size, but found to be between 15 and 20 μm by theinventors of the present invention.

The present invention has as an object an efficient, simple and cheapindustrial process for extracting the oxides contained in vegetalmatters. In a preferred embodiment, the process refers to the extractionof acid oxides.

Another object of the invention are carbon-rich oxide compositionsextracted from vegetal matters. Still another object of the inventionare highly pure oxides extracted from vegetal matters.

These and other objects of the present invention are better understoodby means of the text and examples that follow, the scope of theinvention being limited only by the content of the claims appendedhereto.

The process of the present invention comprises some or all of thefollowing steps:

-   -   A. Hydrolysis of the vegetal matter (acid hydrolysis for the        extraction of acid oxides, basic hydrolysis for the extraction        of basic oxides), at a temperature above 100° C., under        pressure;    -   B. Washing/drying    -   C. Fragilization of the structure;    -   D. Disaggregation of the structure;    -   E. Calcination;    -   F. Milling

More details are now presented, concerning a preferred embodiment of theinvention, not in any way limiting the scope of the invention. Itconcerns the process for the extraction of silica from rice husks, aswell as carbon-rich oxide compositions and pure silica extracted fromvegetal matter.

A. Acid Hydrolysis—Accomplished Under the Following PreferredConditions, without the Exclusion of Any Other:

-   -   temperature range: 100° C.-200° C.;    -   pressure above 2 atmospheres;    -   acid used—sulfuric, hydrochloric or nitric, or their mixture;    -   aqueous acid solution: 3% to 5% of acid, in weight;    -   hydrolysis time: 30 minutes to 2 hours;    -   ratio between weight of vegetal matter to weight of acid        solution: 2:1 to 1:4.

During the hydrolysis potassium, sodium, calcium, magnesium and otherinorganic impurities react with the acid forming, for instance, solublesulfates with the sulfuric acid. Also during the hydrolysis the aciddecomposes the hemicellulose.

When the process of the invention aims at obtaining basic oxides, thehydrolysis is a basic hydrolysis, and the preferred conditions remainthe same, except that alkali is used instead of acid, and the preferredalkali are ammonium hydroxide, potassium hydroxide and sodium hydroxide,or their mixture.

B. Washing/Drying

The resulting material from the hydrolysis is washed with water, in oneor more operations aimed at this end, depending on the desired purity ofthe silica. One aims at the removal of soluble salts and the decomposedhemicellulose. One can use soft water—that is, with low salt content—,distilled water or deionized water. The purity of the silica, asobtained at the end of the process depends on the efficiency of thesoluble salt removal generated in the previous step, avoiding thepresence of salts of basic character which later, during the step ofcalcination, form silicates that favor the sinterization of thesilica—from this angle the use of hard water is less adequate. Somepreferential conditions during washing are as follows:

-   -   the washing operation starts with potable water and ends with        distilled or deionized water;    -   the washing operation is performed until the pH of the resulting        waters is about 6;    -   a larger number of washing actions with smaller amounts of water        is preferred to a smaller number of washing actions with larger        amounts of water.

At this stage one also performs, before going on to the next steps, theelimination of excess water from the washed material, for instance byfiltration and/or drying, in any of the ways known to one skilled in theart.

It has been verified that the use of potable water provided silica withBET specific area up to about 260 m²/g, while the use of distilled ordeionized water considerably increased such BET area to values above 480m²/g

BET (after Brunauer, Emmett e Teller) refers to a measure according thedescription by Paul A. Webb, P. A. and Orr, C, in Analytical Methods inFine Particle Technology, edited by Micromeritics Instrument Corporation(One Micromeritics Drive, Norcross, Ga. 30093, USA, ISBN 0-9656783-0-X).

C. Fragilization of the structure—this step aims at providing fragilityto the obtained material, making its later disaggregation easy andefficient, also allowing the calcination temperature to be lower(compared to the absence of this step) as part of the organic materialbegins to decompose here, under gaseous form. Such gases can be utilizedas fuel in later steps of this process, for instance during calcination.

An efficient fragilization means is the exposure to heat between about270° C. and the temperature just below the flash point, or flame formingtemperature of the vegetal matter from the prior step (ordinarilybetween about 410° C. and 430° C.), for about two hours, or until a deepdark color is obtained, or until the material becomes brittle or easilybreakable. Typically, the temperature of this step is about 320° C. Thetime necessary to perform this step depends of the husk heating process,being shorter when the husks are carried by a flux of hot air.

The fragilization step renders easier the later milling operation, andallows the use of lower temperatures during the later calcinationoperation (compared to the temperatures used in the absence of the stepof fragilization).

D. Disaggregation—the material originated from the previous step can beeasily disaggregated by any adequate physical, physico-chemical orchemical action. It is typically a mechanical operation, such a milling,advantageously under dry conditions. It was verified to be adequate,without excluding any alternative, a dry milling with 5 mm diameterceramic spheres, providing a fine powder with average diameter particlepassable through a 325 mesh tamis or smaller. Also adequate are deviceswith mechanical or ultrasound vibrating means, crushing means, grindingmeans, or any equivalent means capable of performing disaggregation.

This disaggregation operation contributes to inhibit the aggregation ofthe silica in a later calcination step.

For some industries the resulting material from this step is useful,namely a composition essentially comprising carbon and silica, in aratio of about 60:40. A example is the tire industry, as the rubbercomposition comonly comprises both carbon black and silica. Thiscomposition also finds uses in the cement industry, paint industry, etc.

Performing steps C and D sequentially is advantageous as they favor afiner particle size, compared to the alternative when one or the otheris absent.

E. Calcination—it seeks to eliminate the organic part in the vegetalresidue with the lowest possible temperatures, or with the shortestresidence times of the dust in the calcination equipment. The higher thecalcination temperature, the greater the undesired aggregation of thesilica particles, mainly if alkaline or alkaline-terrous impurities arepresent. The preferred calcination temperature is between the flashpoint of the husk (between about 410° C. and 430° C.) and about 900° C.,more specifically between about 440° C. and 850° C., and moreadvantageously between 500° C. and 650° C. An adequate form ofcalcination is the continuous burning of the dust injected along withfuel in the flame of a blast burner or blowpipe, directed to theinterior of the burning chamber to accomplish the burning in theshortest possible time. One aims at obtaining a white silica dust at thelowest possible temperature that is capable of eliminating all carbon;from another angle, when the final product is white, one knows that theorganic content has been substantially eliminated. When one burns thematerial obtained according to the previous steps, one favors the directprovision of an amorphous silica dust of high specific area, lowaggregation and low average particle size, of about 5 μm.

Other adequate ways to perform the calcination is by using a fluidizedbed oven, a rotary oven as the ones presently employed to burn ceramicdusts, a muffle type furnace with shallow crucibles, etc.

F. Milling—this step aims at providing the adequate finer particle sizeto the amorphous silica, as required to specific needs. It includes anyoperation that decreases the particle size obtained in the previoussteps. It can be by wet means—for instance a ball mill with a deflockingagent—or by dry means, for instance by the chock of opposing jets, animpact rotary mill with particle size selection, a ball mill withcontinuous sweep of the fine fraction, etc.

One of the advantages of the process of the invention is, by comprisingthe steps of fragilization and disaggregation, permiting a lower use ofenergy during calcination, if compared to either the traditional directburning of rice husks or to the process revealed by Real, Alcalá andCriado, previously mentioned. Furthermore, the use of higher pressuresduring the hydrolysis step permits a shortened hydrolysis time and theuse of a lower acid content solution, favoring—or at least notdecreasing—qualities such as purity, BET specific area and smallparticle size in the final product.

One of the processes of the invention is the one that comprises steps A,B, C, D and E described before, for obtaining high purity (above 99%)amorphous silica, with particle size between about 1 to 5 μm, highspecific area above at least about 260 m²/g, and high chemicalreactivity.

Another process of the invention is the one that comprises steps A, B,C, D, E and F, through which one obtains amorphous silica with particlesize below 1 μm.

Another process of the invention is the one that comprises alternativelysteps A, B, C and E, or A, B, D and E, either one optionally followed bystep F, for obtaining high purity amorphous silica.

Another aspect of the invention is a high purity oxide, preferablysilica, extracted from vegetal matter according to any of the processesmentioned hereinbefore.

Another process of the invention is the one that comprises steps A, Band C for obtaining carbon-rich oxide compositions, optionally followedby steps D and/or F.

Another aspects of the invention is compositions comprising essentiallycarbon and silica, obtained through a process according to steps A, Band C, optionally followed by steps D and/or F.

Examples will now be given, only as illustrations of the invention, tofacilitate its understanding.

Three batches identified as 1, 2 and 3, each with 30 kg of rice husks ofdifferent origins are subject to the following steps:

A. Hydrolysis

-   -   batches 1 and 2-4.5% sulfuric acid aqueous solution, using a 1:1        ratio between the weights of husks and acid solution, pressure        of 5 atmospheres, temperature of 150° C., during 1 hour.    -   batch 3-5% hydrochloric acid aqueous o, using a 1:1 ratio        between the weights of husks and acid solution, pressure of 7        atmospheres, temperature of 170° C., during 2 hours.        B. Washing/Drying    -   batches 1 and 2—potable water, in a container provided with        filtration means, 30 liter portions for each washing action        under agitation, until the resulting waters have a pH of about        6, followed by drying during 1 hour at 120° C. in a muffle type        oven.    -   batch 3—the same as for batches 1 and 2, including two extra 30        liter distilled water washing actions.        C. Fragilization    -   all batches—heating to 320° C. during 60 minutes, with the        material disposed in thin layers inside crucibles, in a muffle        type oven.        D. Disaggregation    -   all batches—dry milling in a rotary mill with 5 mm diameter        zirconia spheres, for 8 hours. The material obtained is a deep        dark powder with average particle size below 44 μm.        E. Calcination    -   batches 1 and 2—temperature 550° C. in a fluidized bed oven,        residence time 2 hours.    -   batch 3—temperature 500° C. in a fluidized bed oven, residence        time 2 hours.        F—Mixing    -   batches 1 and 2—milling with 5 mm diameter zirconia spheres, in        a wet medium with 2% in weight of ammonium polyacrylate as        deflocking agent, for 2 hours.    -   batch 3—same as with other batches, for 6 hours.

RESULTS

The table below shows the results:

particle (*) color and % size specific area % average Batch purity inSiO₂ (μm) (BET m²/g) Pozolanicity 1 (before step F) white, 99.2 5 260 901 (after step F) white, 99.2 1 260 90 2 white, 99.6 0.8 280 90 3 white,99.8 0.7 420 99 (*) According to the process described by Chapelle, J.in Revue de Matériaux de Construction, vol. 512, pages 136 andfollowing. 1958, France.

1. A process for the extraction of oxides contained in vegetal matter toobtain high purity oxides, comprising the following steps: A. AcidHydrolysis of the vegetal matter at a temperature between 100° C. and200° C., pressure above 1 atm and acid concentration in water between 3%and 5% by weight; B. Washing and drying the hydrolyzed materialresulting from step A; C. Fragilization of the dried material resultingfrom step B; D. Disaggregation of the fragilized material resulting fromstep C; E. Calcination of the disaggregated material resulting from stepD.
 2. The process according to claim 1, further comprising an additionalfinal milling step.
 3. The process according to claim 1, wherein thehydrolysis step obtains acid oxides utilizing the following conditions:an acid selected from the group consisting of sulfuric acid,hydrochloric acid, nitric acid, and combinations thereof; and ratiobetween the weights of vegetal matter and acid solution from about 2:1to about 1:4.
 4. The process according to claim 1, wherein said washingstep B is performed with water selected from the group consisting of:potable or soft water, distilled water and deionized water, until thewater resulting from the washing has a pH of about
 6. 5. The processaccording to claim 1, wherein said fragilization step C is performed byexposure to heat between about 270° C. and the flame forming temperatureor the flash point of the washed and dried material, about 320°C., forup to about 2 hours.
 6. The process according to claim 1 wherein saiddisaggregation step D is a mechanical operation.
 7. The processaccording to claim 1 wherein said calcination step E is performedbetween the flame forming temperature or the flash point of saiddisaggregated material resulting from step D and about 900° C.
 8. Theprocess according to claim 1 wherein said calcination is performed as acontinuous burning of dust, injected along with fuel in the flame of ablast burner or blowpipe.
 9. The process according to claim 6, whereinsaid mechanical operation is a dry milling operation.
 10. The processaccording to claim 1 wherein said calcination step E is performedbetween about 440° C. and about 850° C.
 11. The process according toclaim 1 wherein said calcination step E is performed between about 500°C. and about 650° C.